Be on the lookout for aphids

Agronomist, Paul Parker (NSW DPI), has reported aphids present in several canola crops in parts of the Riverina district in New South Wales. It is likely that they are cabbage aphids (Brevicoryne brassicae), which have a dull grey-green coloured body and grow up to 3 mm in length. Paul says the affected crops are under moisture stress and beginning to flower. Cabbage aphids attack canola and may also infest cruciferous forage crops. Like all aphid species, cabbage aphids feed by sucking sap from plants using their specialized mouthparts. Aphids also secrete honeydew, which can lead to a black sooty mould forming when dense colonies occur. This can decrease the ability of the plant to photsynthesise.

Click here for images of cabbage aphids and refer to PestFacts Issue No. 7 for more information.

Pulse Development Officer, Trevor Bray (Pulse Australia), reports finding aphids in a vetch crop near Sea Lake, in the Mallee district of Victoria. Based on the description provided, they are likely to be cowpea aphids (Aphis craccivora), which are commonly found on faba beans, lentils, medics, lupins and vetch. Trevor says both winged and wingless aphids were present and that occasional plants had been colonised. Aphid numbers were relatively low and the plants were not visibly stressed, however a continuation of warm weather conditions could lead to a rapid increase in the population size.

Adult cowpea aphids are shiny black and nymphs are dull grey in colour. All stages have white and black coloured legs. Dense colonies can develop on individual plants, or in well-defined patches. Infestations start on the growing tips, and spread down the stem, causing leaf bunching and stem twisting. Cowpea aphids tolerate warm dry weather, and can be severe on water stressed plants. Cowpea aphids are also known to be vectors of many important plant viruses.

Click here for images of cowpea aphids.

Although these reports are quite early in the season, they should serve as a timely reminder to be on the lookout for aphids in crops, particularly as the weather becomes warmer. Winged aphids move into crops in autumn from alternative host plants; usually from weeds found on roadsides and verges where they over-summer. They build up on crop edges before moving throughout the crop. Aphid survival, development and population growth are influenced by temperature. Spring triggers a rapid increase in their life cycle and numbers can build up quickly within a crop.

Monitoring of crops for aphids should include checking representative parts of the entire paddock. Check at least five points in the paddock and look for aphids on a minimum of 20 plants at each point. Crops should also be monitored for beneficial insects, and these should be considered before deciding on control strategies. Beneficial insects include parasitic wasps, ladybirds, hoverflies and lacewings. They are usually prevalent in spring and provide a reliable form of biological control when low to moderate numbers of aphids are present. If chemical control is warranted, selective insecticides (such as pirimicarb) are available, which are aphid specific and less harmful to beneficial insects.